Conventional systems for tracking and/or monitoring assets (herein generally referred to as “asset tracking systems”) utilize wireless tags that generally respond to any broadcast that is made. The wireless tags usually are passive, and the responses that the passive wireless tags make are often referred to as “chirps.”
More sophisticated asset tracking systems utilize semi-passive wireless tags and/or active wireless tags. A semi-passive wireless tag includes an internal power source for transmitting, and an active wireless tag includes an internal power source for both receiving and transmitting. Semi-passive and active wireless tags generally have greater capabilities than passive wireless tags due to the internal power sources. Of course, power consumption is always a concern when a wireless tag includes an internal power source, since the internal power supply limits the useful life of the wireless tag, after which time maintenance is required (e.g., replacement of the internal power source).
In improved asset tracking systems, such as those disclosed in the above patent applications and patents that have been incorporated herein by reference, a wireless tag responds to a broadcast if the broadcast includes a common designation matching a common designation of the wireless tag. Such a common designation may comprise, for example, an “asset class” associated with the wireless tag. Ad hoc networks further may be created based on such classes, which ad hoc networks are referred to as “class based” networks.
Class based networks (and common designation networks in general) are beneficial because, in such networks, a communication device, such as a wireless tag, generally only transmits a response to a broadcast if the broadcast includes a class (or common designation) that matches a class (or common designation) of that communication device. Indeed, in a communication device employing a wakeup sequence of one or more of the patent references incorporated herein by reference, such communication device does not even process a broadcast once it is determined that the broadcast fails to include a matching class of the communication device. Consequently, the internal power supply of a semi-passive or active communication device is not drained by needless processing and/or responses to broadcasts.
In asset tracking systems, it often is important to know the physical location of an asset. This could include knowing where the asset is within a limited physical area, such as a warehouse; this also could include knowing where the asset is within several different warehouses in several different geographical locations, as well as where the asset is during transit between such locations.
In a conventional asset tracking system in which communication devices comprising semi-passive or active radios are placed on the assets, a conventional method for acquiring visibility of the assets includes broadcasting within an area at regular intervals to solicit responses from all of the radios within the area. The responses from the radios reveal the radios, and thus the assets, that are in the area.
This method is not advantageous because the regular, repetitive broadcasts result in an unnecessary power drain of the responding radios. Interference also can occur if a large number of radios respond at the same time, thereby making it difficult to accurately identify all of the radios within the area that respond to the broadcast.
In an alternative conventional method, a timer is included with each radio and the radio is configured to transmit at periodic intervals as a function of the timer. The radio thereby alerts the tracking system as to the whereabouts of the radio and, thus, the asset with which it is associated. By including timers with each radio, the radios may transmit at differing times in order to avoid unnecessary interference. A radio also can be set to sleep between intervals and to be awoken by the timer for making its regular transmissions. This increases the useful life of the radios because the radios do not consume power by actively listening for broadcasts while sleeping.
This alternative method permits determinations as to the delivery and continued presence of an asset at a particular area. Nevertheless, this alternative method does include drawbacks. For instance, by using timers, the radios are inaccessible by the asset tracking system during the sleep periods. Another drawback is that the radios automatically awake and transmit without regard for their location and without regard for whether the transmission is actually warranted or even desired. In this respect, during transportation on a plane, a radio may awaken and transmit, which may cause unwanted interference with the operation of the airplane. Preprogrammed transmission at regular intervals also may reveal the presence of the asset to unauthorized persons snooping for such radio transmissions.
Accordingly, better asset tracking systems and methods are desired that minimize unnecessary power consumption and that reduce unnecessary transmissions by communication devices associated with assets.